The invention relates to a "hands free" telephone set in which, between a terminal of the transmission path and a terminal of the receiving path a first path is formed, which comprises at least the coupling circuit leading to the subscriber line and a variable-gain amplifier, a second path being connected between the two terminals of the first path and comprising at least a second variable-gain amplifier which is controlled by a linear regulator so as to keep its output signal constant as a function of its input signal, the gain of the said amplifier of the first path being responsive to the gain of the said amplifier of the second path, amplifier and/or attenuation means being provided in the one and/or the other path to ensure that the gain in the second path remains higher than the gain in the first path in the frequency band where the loop oscillations may be produced.
In such a telephone set disclosed in Applicants copending application Ser. No. 372,430, filed Apr. 28, 1982 sustained amplitude oscillations which are controlled by the linear regulator so as to render them non-annoying and in particular inaudible are produced in the loop which comprises the acoustic path between the loudspeaker and the microphone of the set and which is closed by the said second path; at the same time annoying sustained oscillations, usually referred to as Larsen oscillations are prevented from being produced in the electro-acoustic loop which is closed by the first path and comprises the coupling circuit of the set, as a result of imperfections in this coupling circuit or as the result of an acoustic coupling in the remote subscriber set.
In other types of "hands free" telephone sets, for example the set described in U.S. Pat. No. 4,162,374 there is used, to prevent the annoying Larsen oscillations from being produced, a different procedure which employs the gain change-over by speech and in general requires a detection of the speech signals in the transmission path and in the receiving path of the set to determine in which path the speech signal is stronger; a gain increase control is effected in this path and a gain decrease control is effected in the other path, these two gain variations being complementary so as to ensure that the total gain in the loop where the Larsen oscillations are liable to be produced remains less than unity.
Although the system described in the above-mentioned patent application Ser. No. 372,430 does not employ the gain change-over by speech in order to avoid the annoying Larsen oscillations; it may be useful for other reasons to effect a gain change-over manually or controlled by the speech. For example when the "hands free" set is used in a very noisy environment it may be important to increase the gain in the receiving link in order to increase the listening comfort, and to decrease the gain in the transmission link, even if this would mean that a speaker must speak louder or closer to the microphone. This gain change-over may be initiated manually or directly controlled by the speech. In this second assumption, when a speaker does not speak into the "hands free" set, the gain in the receiving link must be increased, which increases the listening comfort and the gain in the transmission link must be decreased, which avoids that the other speaker hears at a comparatively loud level the ambient noise in the environment where the "hands free" set is situated. If in contrast therewith, when the speaker speaks into the "hands free" set the gain in the transmission and receiving links must again have their nominal values.
The system disclosed in the patent application Ser. No. 372,430 now renders it unexpectedly possible to provide an extremely simple means to effect these gain change-overs.